Calcium carbonate precipitation method

ABSTRACT

A method for producing precipitated calcium carbonate by reacting an aqueous solution of calcium nitrate [Ca(NO 3 ) 2  ] with an aqueous solution of ammonium carbonate [(NH 4 ) 2  CO 3  ] and allowing calcium carbonate to precipitate from the resultant mixture containing nitrate [NH 4  NO 3  ] in the mother liquor, the process being characterized in that: (i) the calcium nitrate [Ca(NO 3 ) 2 )] solution utilized in the processes is prepared by slaking lime [CaO] in water in the presence of ammonium nitrate [NH 4  NO 3  ] to form calcium nitrate [Ca(NO 3 ) 2  ] and ammonium hydroxide [NH 4  OH] in solution, filtering the solution to render it solids free, and heating the filtrate to dissociate the ammonium hydroxide [NH 4  OH] and to drive ammonia gas [NH 3  ] from the solution; (ii) the ammonium carbonate (NH 4 ) 2  CO 3  solution utilized is prepared by absorbing ammonia gas [NH 3  ] and carbon dioxide gas [CO 2  ] in water, the ammonia gas preferably being derived from the step in (i) above in which the Ca(NO 3 ) 2  solution is heated; and (iii) the ammonium nitrate used is derived from the precipitation phase during which calcium carbonate is precipitated from the mother liquor containing ammonium nitrate.

FIELD OF THE INVENTION

This invention relates to methods for producing crystalline materials byprecipitation. According to one aspect of the invention it relates tothe production of precipitated calcium carbonate. The invention thusrelates particularly, but not exclusively, to the production of highpurity calcium carbonate from relatively impure calcium source material.

BACKGROUND TO THE INVENTION

Calcium carbonate is a natural carbonate which is present in largequantities particularly in calcitic and dolomitic limestone. In rawimpure form these products are used in various industries including themining and cement industries. In purified crystalline form calciumcarbonate is used in the production of paint, in toothpaste, papercoating and sizing and as an acid neutraliser or as a filler inpharmaceutical products, amongst other applications.

Despite the natural abundance of calcium carbonate as such pure calciumcarbonate is a relatively expensive product due to the difficulties andexpenses associated with conventional purification methods.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a new method forproducing precipitated calcium carbonate from a natural source materialcontaining calcium values.

EP-A-49666 [Kalk Chemische Fabrik] teaches the preparation of calciumcarbonate [vaterite] by reaction of a calcium ion containing aqueoussolution with carbonate ions containing aqueous solution. Preferred arethe use of a solution containing calcium nitrate and a solution ofammonium carbonate.

Chemical Abstracts, AN: 116:258510H of CN-A-1055718 [Zou et al]discloses the preparation of calcium nitrate from calcined [dolomite]ores [i.e. lime] by impregnation of the calcination product withammonium nitrate solution. The resultant calcium nitrate solution isused for the preparation of calcium carbonate by carbonisation with CO₂-containing air.

WO-A-7900931 [Anderson] discloses a process for the preparation of whiteliquor for the sulphate pulping process, wherein quicklime is slakedwith green liquor and the slaked lime formed is reacted with the sodiumcarbonate in the green liquor to sodium hydroxide in a causticizingprocess, the lime sludge formed being separated. According to thatinvention, the process from the slaking of the lime up to the separationof the white liquor and the lime sludge is carried out at a pressureabove the atmospheric pressure. In a preferred embodiment, the processis also carried out at a temperature above the normal boilingtemperature of the system.

SUMMARY OF THE INVENTION

According to the invention there is provided a method for producingprecipitated calcium carbonate by reacting an aqueous solution ofcalcium nitrate [Ca(NO₃)₂ ] with an aqueous solution of ammoniumcarbonate [(NH₄)₂ CO₃ ] and allowing calcium carbonate to precipitatefrom the resultant mixture containing nitrate [NH₄ NO₃ ] in the motherliquor, the process being characterised in that

[i] the calcium nitrate [Ca(NO₃)₂ ] solution utilised in the processesis prepared by slaking lime [CaO] in water in the presence of ammoniumnitrate [NH₄ NO₃ ] to form calcium nitrate [Ca(NO₃)₂ ] and ammoniumhydroxide [NH₄ OH] in solution, filtering the solution to render itsolids free, and heating the filtrate to dissociate the ammoniumhydroxide [NH₄ OH] and to drive ammonia gas [NH₃ ] from the solution;

[ii] the ammonium carbonate (NH₄)₂ CO₃ solution utilised is prepared byabsorbing ammonia gas [NH₃ ] and carbon dioxide gas in water, theammonia gas preferably being derived from the step in [i] above in whichthe Ca(NO₃)₂ solution is heated;

[iii] the ammonium nitrate used is derived from the precipitation phaseduring which calcium carbonate is precipitated from the mother liquorcontaining ammonium nitrate.

The precipitation reaction between the two reagent solutions which reactto form the precipitated CaCO₃ is preferably performed by feeding thesolutions into intimate contact with one another in a contact zone of areactor arrangement and immediately displacing the mixture from thecontact zone, and allowing the resultant crystalline reaction product toprecipitate from its mother liquor and separating the precipitatedcrystals from the mother liquor.

In a preferred form of the invention the reagent solutions areintroduced into a reactor arrangement which is in the form of anelongated tubular conduit into which the reagent solutions are fed undera pressure of between 1 and 5 bar gauge pressure. It has been found thatthe application of feed pressure in this range results in a decrease incrystal particle size.

In one application of this aspect of the invention there is thusprovided a method for selectively producing precipitated calciumcarbonate crystals in which a preferred crystal size distributionpredominates, the method comprising the steps of intimately contactingan aqueous solution of Ca(NO₃)₂ with an aqueous solution of (NH₄)₂ CO₃whilst controlling the pressure of the reaction to remain within a rangeat which the formation of the required particle size and distribution ofthe CaO₃ crystals is favoured, allowing the resultant crystallinereaction product to precipitate from its mother liquor and separatingthe precipitated crystals from the mother liquor.

The method may be used to produce precipitated calcium carbonate whichis predominantly of the vaterite crystalline form which has been foundto be the default crystalline form when the temperature was kept between15° C. and 75° C. The reaction is for this purpose most preferablycarried out at between 34° C. and 40° C.

With residence in slurry form the vaterite crystal form has been foundto convert into calcite and is known in the art.

According to a further aspect of the present invention there is provideda method for producing precipitated calcium carbonate crystals of aselected crystal size comprising the steps of reacting an aqueoussolution of calcium nitrate with an aqueous solution of ammoniumcarbonate and controlling the crystal size of the precipitated calciumcarbonate by providing at least one of the reagent solutions at aconcentration favouring the formation of the desired crystal size,allowing the resultant crystalline reaction product to precipitate fromits mother liquor and separating the precipitated crystals from themother liquor.

Such concentration control for influencing crystal size may of course becombined with reaction pressure control as described above.

In one embodiment of this aspect of the invention it has been found thatby reacting 20% [by mass] solution of Ca(NO₃)₂ with a stoichiometricequivalent quantity of (NH₄)₂ CO₃ it is possible to produce calciumcarbonate crystals having a d₅₀ value of less than 6 microns, and thatan increase in the concentration of the Ca(NO₃)₂ leads to the formationof finer crystals.

The various aspects of the invention will now be illustrated withreference to the accompanying examples:

EXAMPLES OF THE INVENTION Example 1 Selective Preparation of DifferentCaCO₃ Crystalline Forms

[a] Preparation of Stock Solutions

50 Kg NH₄ NO₃ was dissolved in 120 liters water with stirring, to whichwas added 17.5 kg lime sourced from Lime Acres in South Africa. 100 mlof triethanolamine was added to the dissolved NH₄ NO₃ before theaddition of the lime. The mixture was filtered to remove undissolvedsolids. The clear Ca(NO₃)₂ /NH₄ OH filtrate was transferred to aseparate vessel and heated to a temperature of 80° C. and held at 80° C.for 30 minutes to drive off ammonia gas. [Care must be taken to ensurethe pH of the Ca(NO₃)₂ solution remains above 9.5].

A solution of approximately 10% (NH₄)₂ CO₃ was prepared in 100 liters ofwater by absorbing NH₃ [released by heating the filtered Ca(NO₃)₂ /NH₄OH solution as described above] and pure CO₂ gas in the water in anabsorbtion column.

The stock solutions of Ca(NO₃)₂ and (NH₄)₂ CO₃ as prepared above wereused to demonstrate the invention.

It will be appreciated that the ammonium carbonate solution may in thealternative have been prepared by feeding ammonia gas from anothersource with CO₂ gas into a scrubber arrangement to absorb such gases inwater thereby to render a solution of ammonium carbonate.

[b] Influence of Reaction Pressure

The stock solutions prepared as described above were fed by means ofconstant displacement pumps at various pressures into an elongatedreaction chamber of about 5 m in length. It was found that the pressurein the reaction chamber, which was controlled by means of a valve at theoutlet end thereof, had a marked influence on the size of the crystalsof CaCO₃ formed by the reaction between the concentrated stocksolutions. Reaction temperatures were measured by determining thetemperature of the final reaction slurry at the end of the reactionchamber. The results of typical pressure and temperature influences oncrystal size are summarised in Table 1, the crystal size values beingthe d₅₀ values in microns.

                  TABLE 1                                                         ______________________________________                                        Effect of Pressure:                                                                           GAUGE PRESSURE [BAR]                                          REACTION TEMPERATURE                                                                            0        3        5                                         ______________________________________                                        17° C.     ND       2.44     1.86                                      50° C.     7.27     2.38     2.38                                      ______________________________________                                    

It will be seen that changes in reaction pressure affects particle sizeand distribution specifically, pressure above zero bar reduces particlesize and narrows distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

The tightening or narrowing effect of the present invention on particlesize distribution is graphically illustrated in FIG. 1.

In FIG. 1 Graph A represents the particle size distribution andcomposition ranges for a commercially available ground calciumcarbonate.

Graph B represents the same for a precipitated calcium carbonateproduced by the method of the invention at 50° C. and 3 bar pressure.

It will be seen that the product of the method of the invention displaysa sharper gaussian distribution of crystal sizes than the commercialproduct.

[c] Influence of Concentration of Feed Solutions

It has been found that increased concentration from 20% Ca(NO₃)₂ to 30%Ca(NO₃)₂ typically results in a decrease in mean particle size, e.g. d₅₀6 μm to d₅₀ 2 μm.

Example 2

The purity of the product produced by the method as described in Example1[a] and [b] is extremely high and is in many respects comparable withthe industry spectrographic standard for CaCO₃ marketed by JohnsonMatthey. In Table 2 below the relative values for various impurities arecompared against the Johnson Matthey standard [taking the quantities ofimpurities in such standard as being 1] and against two commerciallyavailable products identified as A and B.

                  TABLE 2                                                         ______________________________________                                                                        Product of                                             JM  A           B      Invention                                     ______________________________________                                        SiO.sub.2  1     2,8         2,7  0,61                                        Al.sub.2 O.sub.3                                                                         1     1,1         2,5  0,67                                        Fe.sub.2 O.sub.3                                                                         1     2,2         3,5  0,83                                        Mn.sub.2 O.sub.3                                                                         1      1,16        1,37                                                                              1,08                                        MgO        1     19,8        17,7 1,03                                        P.sub.2 O.sub.5                                                                          1      1,15        0,47                                                                              1,09                                        SO.sub.3   1      0,97        0,89                                                                              2,65                                        Cl.sup.-   1      1,17        1,15                                                                              1,41                                        K.sub.2 O  1      1,33        1,76                                                                              1,93                                        Nm.sub.2 O 1      1,12        0,61                                                                              1,06                                        ______________________________________                                    

From the above table it will be seen that the product of the inventioncontains only 61% of the quantity of SiO₂ present in spectrographicgrade CaCO₃ while other products in the market contain almost threetimes such amounts. For many of the other impurities similarobservations may be made.

If one bears in mind that the process by which the product of thispurity is achieved starts from very impure CaO, this achievement isremarkable.

I claim:
 1. A method for producing precipitated calcium carbonatecomprising the following steps:(i) preparing an aqueous solution ofcalcium nitrate and ammonium hydroxide by slaking lime in water, in thepresence of an aqueous ammonium nitrate solution; (ii) rendering thesolution of calcium nitrate and ammonium hydroxide solids-free; (iii)heating the solids-free solution of calcium nitrate and ammoniumhydroxide to dissociate the ammonium hydroxide and to drive ammonia gasfrom the solution, thereby producing an aqueous solution of calciumnitrate; (iv) preparing an ammonium carbonate solution by absorbingammonia gas and carbon dioxide gas in water; (v) mixing the aqueoussolution of calcium nitrate produced in step (iii) with the aqueoussolution of ammonium carbonate produced in step (iv); (vi) allowingcalcium carbonate to precipitate from the mixture produced by the mixingof calcium nitrate with ammonium carbonate, as described in step (v),and thereby separating it from a resulting ammonium nitrate-rich motherliquor; and (vii) recycling the ammonium nitrate-rich mother liquor tostep (i).
 2. The method of claim 1, wherein step (v) is performed at apressure of between 1 and 5 bar gauge pressure.
 3. The method of claim1, wherein the calcium nitrate and the ammonium carbonate are fed intointimate contact with one another in a contact zone of a reactor, andthe mixture is immediately displaced from the contact zone, and whereinthe resultant crystalline reaction product is allowed to precipitatefrom the ammonium nitrate-rich mother liquor and the precipitatedcrystals are thereafter separated from said mother liquor.
 4. The methodof claim 3, wherein the reagent solutions are introduced into a reactorwhich is in the form of an elongated tubular conduit.
 5. The method ofclaim 4, wherein the particle size and size distribution is controlledby the use of pressure between 1 and 5 bar gauge pressure.
 6. The methodof claim 5, wherein the ammonia gas for step (iv) is derived from thestep in (iii) above, in which the solids-free solution is heated.
 7. Themethod of claim 1, wherein the ammonia gas for step (iv) is derived fromthe step in (iii) above, in which the solids-free solution is heated. 8.The method of claim 3, wherein the ammonia gas for step (iv) is derivedfrom the step in (iii) above, in which the solids-free solution isheated.
 9. The method of claim 4, wherein the ammonia gas for step (iv)is derived from the step in (iii) above, in which the solids-freesolution is heated.
 10. The method of claim 4, wherein the particle sizeand size distribution of the calcium carbonate produced by the processis controlled by controlling the concentration(s) of one or both of thesolutions mixed in step (v) of the method.
 11. The method of claim 10,wherein a 20 percent (by mass) solution of calcium nitrate is reactedwith a stoichiometric equivalent quantity of ammonium carbonate toproduce calcium carbonate crystals having a d₅₀ value of less than 6microns.
 12. The method of claim 10, wherein the ammonia gas for step(iv) is derived from the step in (iii) above, in which the solids-freesolution is heated.
 13. The method of claim 11, wherein the ammonia gasfor step (iv) is derived from the step in (iii) above, in which thesolids-free solution is heated.